The invention pertains to sealed floating roofs for storage tanks.
Above ground storage tanks are frequently used to store hydrocarbon fluids. Because the stored fluid is volatile, the storage tank is often equipped with a floating roof itself sometimes under a fixed roof. The floating roof floats on the stored fluid and moves up and down with the fluid level. Floating roofs greatly reduce fluid evaporation, preventing loss of the stored fluid and reducing pollution due to hydrocarbon evaporation into the atmosphere.
Additionally, it is desirable to provide floating roofs that are full-contact roofs, allowing the roof structure to be in direct contact with the fluid surface. Such roofs do not allow an air gap between the bottom of the roof surface and contained fluid. When air gaps exist, they allow an evaporation zone over the top of the fluid that increases the risk of leakage around the floating roof structure. However, full contact floating roofs must be tightly sealed to prevent direct vapor leakage or evaporation through small gaps in the roof structure. Such leakage or evaporative losses can create unwanted pollution as well as the economic loss of stored product.
However, typical floating roofs are large and must be assembled on site. The structure typically comprises a framework of segments that are assembled to form and open lattice, and a plurality of roof panels or sheets that are then attached to the framework to form the upper surface of the roof. Roof panels or sheets are typically attached by bolting them in place, and sometimes sealed with sealants that are generally known in the industry.
Such construction can prevent the desired sealing effectiveness of the roof from being achieved. Sealants can degrade over time due to environmental conditions, and may be attacked by contact with the stored fluid or by vapors. Bolted connections are not vapor tight. Effecting repairs can require draining the storage tank, resulting in lost revenue, and may require workers to operate in a hazardous environment. Accordingly it is desirable to provide a seal between the roof panels and the framework that is highly resistant to degradation over time, and that will provide a strong, durable roof over its life expectancy.
One possible approach is to weld the edges of the roof panels to the framework, so that every seam between the roof panels and the framework is permanently sealed. However, if welding were to be done by hand, the large size of the typical floating roof would require a large expenditure of man hours, and the work would often have to be done in an extremely uncomfortable environment.
Accordingly, it is desirable to provide a floating roof, especially one intended for full contact, with welded construction so that the roof panels are welded to the framework. It is further desirable to accomplish this welding with an automatic welder.